The development of tunable solid state lasers based on the Cr4+-ion started in 1988 with forsterite, Cr:Mg2SiO4 [V. Petricevic, S. K. Gayen and R. R. Alfano, Appl. Phys. Letters 53 (1988) 2590]. It was extended to other crystalline media, such as Cr4+-doped Y3Al5O12 [A. P. Shkadarevich, in: OSA Proceedings on Tunable Solid State Lasers, Ed. M. L. Shand and H. P. Jenssen (Optical Society of America, Washington, D. C., 1989), Vol. 5, pp. 60-65], Y2SiO5 [J. Koetke, S. Kuck, K. Petermann, G. Huber, G. Gerullo, M. Danailov, V. Magni, L. F. Qian, and O. Svelto, Opt. Commun. 101 (1993) 195], and Y3ScxAl5-xO12 [S. Kuck, K. Peterman, U. Pohlmann, U. Schonhoff, and G. Huber, Appl. Phys. B58, (1994) 153]. These materials also typically include the chromium dopant within the crystalline structure in other valence states. These other valence states can act as a trap and reduce the total concentration of Cr4+ lasing ions. Increasing Cr4+ lasing ion concentration in the laser materials can improve efficiency of laser devices.
Different approaches have been used to increase Cr4+ concentration in crystals. For example, specific crystal growth conditions have been created using different total amounts of chromium oxide in the initial charge, different oxygen content in the growth atmosphere, and/or different after-growth heat treatments. Possible mechanisms for chromium ion incorporation in a crystal structure, with appropriate charge compensation, have also been discussed (W. Chen and G. Boulon, Growth mechanism of Cr:forsterite laser crystal with high Cr concentration, Optical Materials. 24 (2003) 163-168; R. Feldman, Y. Shimony and Z. Burshtein. Dynamics of chromium ion valence transformations in Cr,Ca:YAG crystals used as laser gain and passive Q-switching media Optical Materials, Volume 24, Issues 1-2, October-November 2003, Pages 333-334; J. L. Mass, J. M. Burlitch, S. A. Markgraf, M. Higuchi, R. Dieckmann, D. B. Barber and C. R. Pollock, Oxygen activity dependence of the chromium (IV) population in chromium-doped forsterite crystals grown by the floating zone technique, Journal of Crystal Growth, Volume 165, Issue 3, 1 Aug. 1996, Pages 250-25).